1. Field of the Invention
The present invention relates to a resin sealing method and apparatus for forming a package by sealing a lead frame with an electronic part such as a semiconductor chip with a resin.
2. Description of the Conventional Art
In recent years, the demands for miniaturization of an electronic device such as a portable device have been on the increase. To meet such demands, packages are rapidly reduced in size and weight. Small packages with a flat bottom surface and an external electrode on its side, called a QFN (Quad Flat Non-leaded), are widely used.
Now, a conventional method of manufacturing a QFN package will be described. First, a semiconductor chip is placed on a die pad of a lead frame, and the pads of the semiconductor chip and the lead frame are bonded by a wire. Then, the lead frame with the semiconductor chip is placed on a lower mold. With the semiconductor chip received in a cavity of an upper mold, the upper and lower molds are closed and the lead frame is clamped. Then, a molten resin is poured into the cavity for curing. Thereafter, the molds are opened and a resin mold part, having the integrated semiconductor chip, lead frame and cured resin, is removed, and a package is completed by punching. In the completed package, the external electrode is exposed from the side and lower surface of the cured resin. As such, the QFN package has an advantage of a reduced area or real estate since the lead is inside the package when viewed from above.
However, in the above described conventional manufacturing method, the cured resin may protrude from the bottom surface of the package which should be flat, as a result of resin sealing. Such cured resin may be formed by the following process. Namely, due to foreign matters adhering to the mold face of the lower mold, the lower surface of the lead frame and the like, or a slight curve or swelling of the lead frame, a space is formed between the mold face of the lower mold and the lower surface of the lead frame. If the molten resin pours onto the lower surface of the lead frame through the space between the lead frame and the lead or between the lead and the die pad, a protrusion of the cured resin is formed on the bottom surface of the package. If the lead frame has a slight curve, swelling or the like, the clamping of the lead frame by the upper and lower molds may cause a space between the lower mold and lead frame, disadvantageously allowing a protrusion of the cured resin on the bottom surface of the package.
This results in deterioration of physical appearance and lifting of the package when mounting the package onto a printed circuit board or the like. Consequently, the reliability of an electrical connection between the external electrode of the package and the pad of the printed circuit board decreases and an overall height of the printed circuit board with the mounted package (hereinafter defined as a package mounting height) increases, thereby preventing miniaturization of electronic devices.
To prevent formation of the cured resin protrusion on the bottom surface of the package, a lead frame is used which has a tape on its lower surface. However, this involves more cost. In addition, when the lead frame on a mold is heated by the mold, the difference between the lead frame and the tape in thermal expansion coefficient leads to a curved lead frame, whereby the formation of the cured resin protrusion on the bottom surface of the package cannot be avoided.
An object of the present invention is to provide a resin sealing method and apparatus capable of forming a package with enhanced physical appearance and reliability of an electrical characteristic while restricting a package mounting height by preventing formation of a cured resin protrusion on a bottom surface of the package.
In one aspect, the resin sealing method of the present invention to achieve the above mentioned object refers to a method of sealing a semiconductor chip having a lead frame on its one surface with a resin with use of a mold for resin sealing. The method includes steps of: supplying and setting the lead frame in a prescribed position at a mold face of the mold and enclosing the semiconductor chip on the lead frame in a cavity formed in the mold face of the mold; providing a film with a required heat resistance and elasticity between the opposing mold faces of the mold and applying the film onto the mold face of the opposing mold faces on the other surface side of the lead frame; closing the opposing mold faces of the mold; sealing the semiconductor chip in the cavity with a resin by pouring a molten resin material into the cavity after the closing step; opening the mold and removing a mold part after the resin sealing step; and removing the film. The lead frame has a required degree of freedom i.e., it is not subjected to a pressure from the opposing mold faces of the mold when closing the mold. During resin sealing, the other surface of the lead frame is kept tightly in contact with the film applied to the other surface under a pressure of pouring the molten resin material into the cavity.
In this method, a film is kept in contact with a surface of the lead frame without the semiconductor chip, i.e., a non-mounting surface, for the purpose of preventing a molten resin from pouring onto the non-mounting surface. In addition, foreign matters are prevented from adhering to the film and lead frame. Further, the problem associated with small wrinkles of the film or a curved lead frame is eliminated. Thus, a cured resin protrusion on the bottom surface of the package would not be formed.
In addition, since the side surface of the lead frame on the non-mounting surface side is exposed from the cured resin, adhesion strength in mounting the package onto a printed circuit board and the like increases, thereby facilitating visual inspection.
Therefore, the present invention effectively provides a resin sealing method and apparatus with enhanced appearance and reliability capable of preventing formation of a cured resin protrusion on the bottom surface of the package, restricting a package mounting height, and providing greater adhesion strength in mounting.
In another aspect, the resin sealing method of the present invention refers to a method of sealing a lead frame with a semiconductor chip with a resin with use of opposing upper and lower molds. The method is characterized in that it includes steps of: supplying a resin tape between the upper and lower molds from a supplying roll to a receiving roll; applying the resin tape onto the mold face of the upper mold; placing the lead frame on the mold face of the lower mold with the semiconductor chip received in the cavity of the lower mold; closing the upper and lower molds while preventing the lead frame from being held between the upper and lower molds; pouring a molten resin into the cavity and pressing the lead frame against the upper mold through the resin tape by the poured molten resin; curing the molten resin to provide a cured resin; opening the upper and lower molds; removing a resin mold part with the cured resin and lead frame integrated together; and supplying unused portion of the resin tape between the upper and lower molds while receiving onto the receiving roll used portion of the resin tape, with use of the receiving and supplying rolls.
In the method, a surface of the lead frame without the semiconductor chip, i.e., a non-mounting surface, is pressed against the upper mold by the molten resin through the resin tape, so that an opening used for pouring the molten resin onto the non-mounting surface is closed and the resin tape is compressed and deformed. Since newly supplied resin tape covers the non-mounting surface of the lead frame, foreign matters are less likely to adhere to the surfaces of the resin tape and lead frame. Consequently, formation of the space between the resin tape and the lead frame due to the curving of the lead frame, slight swelling and foreign matters as well as pouring of the molten resin onto the non-mounting surface of the lead frame can be prevented. Thus, a cured resin protrusion would not be formed on the bottom surface of the package. In addition, the side surface of the lead frame on the non-mounting side is exposed from the cured resin since a part of the deformed resin tape is held between the lead frames or between the lead and the die pad.
In still another aspect, a resin sealing method of the present invention refers to a method of sealing a lead frame with a semiconductor chip with a resin with use of opposing upper and lower molds. The method is characterized in that it includes steps of: supplying a resin tape from a supplying roll to a receiving roll between the upper and lower molds; applying the resin tape onto the mold face of the lower mold; placing the lead frame on the lower mold through the resin tape to provide the semiconductor chip directly below the cavity of the upper mold; closing the upper and lower molds while preventing the lead frame from being held between the upper and lower molds; pouring a molten resin into the cavity and pressing the lead frame against the lower mold through the resin tape by the poured molten resin; curing the molten resin to form a cured resin; opening the upper and lower molds; removing a resin mold part with the resin mold and lead frame integrated together; and supplying unused portion of resin tape between the upper and lower molds while receiving used portion of the resin tape onto the receiving roll, with use of the receiving and supplying rolls.
In the method, since the non-mounting surface of the lead frame is pressed against the lower mold by the molten resin through the resin tape, the resin tape is compressed and deformed and an opening allowing the molten resin to pour onto the non-mounting surface is closed. Further, the lead frame is provided with its non-mounting surface facing down above the newly supplied resin tape, whereby foreign matters are less likely to adhere to the surfaces of the resin tape and lead frame. Thus, formation of a space between the resin tape and the lead frame due to the curving, slight swelling and foreign matters of the lead frame as well as the pouring of the molten resin onto the non-mounting surface of the lead frame can be prevented. Thus, a cured resin protrusion would not be formed on the bottom surface of the package. In addition, since a part of the deformed resin tape is held between the leads or between the lead and die pad, the side surface of the lead frame on the non-mounting side is exposed from the cured resin.
The resin sealing method of the present invention is characterized in that the resin tape is preheated by the preheated upper or lower mold in the step of applying the resin tape.
In this method, the lead frame is pressed against the resin tape, which has been thermally expanded due to preheating. Thus, the problem of curving due to a difference in thermal expansion coefficient when the resin tape and lead frame are preliminary applied can be eliminated.
The resin sealing of the present invention is characterized in that the resin tape is an adhesive tape having a base layer and an adhesion layer, the lead frame is pressed against the adhesion layer in the pressing step, and the adhesion tape is removed from the lead frame in or after the opening step.
In this method, adhesion between the adhesive tape and lead frame can be enhanced by the adhesion layer. Thus, the formation of the space therebetween as well as an opening allowing the molten resin to pour onto the non-mounting surface can be more reliably prevented, so that a cured resin protrusion would not be formed on the bottom surface of the package. In addition, since the adhesion layer adheres to the side surface of the lead frame on the non-mounting side, the surface of the lead frame on the non-mounting side is exposed from the cured resin.
The resin sealing method of the present invention is characterized in that it further includes a step of evacuating the space between the upper and lower molds.
In this method, foreign matters are discharged with air through a space between the upper and lower molds, so that adhesion of foreign matters to the surfaces of the resin tape and the lead frame is more reliably prevented, and a cured resin protrusion would not be formed on the bottom surface of the package.
The resin sealing method of the present invention is characterized in that a plurality of semiconductor chips are mounted to the lead frame, a plurality of cavities are provided in the upper mold or the lower mold, the molten resin is poured into the plurality of cavities in the pressing step, and the lead frame is pressed against the resin tape by the molten resin poured into the cavity.
In the method, the molten resin is poured into a plurality of cavities each including a semiconductor chip, whereby the plurality of semiconductor chips are collectively subjected to resin sealing with respect to the lead frame to provide increased workability of resin sealing.
A resin sealing apparatus of the present invention seals with a resin a lead frame placed between upper and lower molds of a mold and having on its one surface a semiconductor chip. The resin sealing apparatus is characterized in that it includes: a means for providing a resin tape between the upper and lower molds; a means for enclosing the semiconductor chip in the cavity of the mold with the other surface of the lead frame in contact with the resin tape and closing the upper and lower molds while preventing the lead frame from being held therebetween; a means for pouring a molten resin into the cavity; a means for closing the upper and lower molds with the poured molten resin cured, where the means for pouring the molten resin presses the other surface of the lead frame against the mold through the resin tape by the poured molten resin.
In the apparatus, the other surface of the lead frame, i.e., a non-mounting surface, is pressed indirectly against the mold by the molten resin with the resin tape therebetween, so that an opening allowing the molten resin to pour onto the non-mounting surface is closed and the resin tape is compressed and deformed. In addition, since the applied resin tape and the non-mounting surface of the lead frame are opposite to each other, foreign matters are less likely to adhere to the surfaces of the resin tape and the lead frame. Thus, formation of a space between the resin tape and lead frame due to the curving, slight swelling and foreign matters of the lead frame as well as the pouring of the molten resin onto the non-mounting surface of the lead frame can be prevented. Thus, a cured resin protrusion would not be formed on the bottom surface of the package. In addition, since a part of the deformed resin tape is held between the leads or between the lead and the die pad, the side surface of the lead frame on the non-mounting surface side is exposed from the cured resin.
The resin sealing apparatus of the present invention is characterized in that it further includes a means for evacuating the space between the upper and lower molds.
With the apparatus, foreign matters are discharged with air, so that adhesion of foreign matters to the surfaces of the lead frame and resin tape is more reliably prevented. Thus, a cured resin protrusion would not be formed on the bottom surface of the package.